In collaborative studies with the Center for Research Education, V.A. Hospital, Los Angeles, the histamine-containing cell in dog stomach has been identified as a small mast cell which contains a specific histidine decarboxylase of low activity and has a histamine content of around 3 pg/cell. These cells are found exclusively in the lamina propria and lie in close proximity to the parietal cell. The cell exhibits the characteristic metachromasia upon staining with toluidine blue. Unlike the mast cell, it is resistant to the histamine-depleting action of compound 48/80. Fractionation of other cells in the dog mucosa by centrifugal elutriation has shown that serotonin and DOPA decarboxylase are present in enterochromaffin cells. In rat gastric mucosa in contrast to dog, histamine resides in an enterochromaffin-like (ECL) cell which contains abundant histidine decarboxylase activity and has no receptor for IgE.